A Doherty amplifier may be used as a power amplifier for amplification of RF-signals for transmission via a transmitter. For accurate and cost-effective transmission of the RF-signals it is desirable that the Doherty amplifier has a high efficiency (useful output power divided by totally consumed power), high linearity (degree of proportionality of output signal to input signal) and low distortions, such as low third order intermodulation (IM3).
The publication “Efficiency of Doherty RF power amplifier systems” by Frederik H. Raab, IEEE Transaction on Broadcasting, Volume BC-33, no. 3, September 1987, discloses a Doherty system, wherein at low output levels the first power amplifier operates as a linear class-B amplifier and the drive to the second power amplifier is cut off.
However, it has been observed that a conventional Doherty amplifier may not have sufficient linearity and sufficient efficiency.
There may be a need for an amplifier, in particular for a power amplifier for amplifying RF-signals, which may exhibit an improved linearity and which may also exhibit an improved efficiency compared to conventional amplifiers.